1. Field of the Invention
The present invention relates to a projector zoom lens and a projector incorporating this projector zoom lens.
2. Description of the Related Art
A front-projection type projector has been widely used nowadays for business presentation or educational purposes in school to project an enlarged image on a screen ahead of the device.
An image display element called light bulb displays an enlarged projected image. Various types such as a liquid crystal panel are now available.
In recent years micromirror devices as a light bulb represented by a digital micromirror device (DMD) have received great attention. It is preferable for a projector zoom lens to be applicable to various types of light bulbs.
In the micromirror device micromirrors are arranged in arrays and they are selectively inclined for image display. The inclination angle of the micro mirrors is about ±10 degrees. By changing the inclination angle, valid reflective light and invalid reflective light are switched. A light source for the micromirror device needs to be provided adjacent to the projector zoom lens so that the lens diameter of the projector zoom lens near the light bulb has to be small. Further, a long back focus is required.
Despite these limitations, the micromirror device is advantageous in downsizing and higher brightness and will be in widespread use.
The projector zoom lens mounted in the projector is required to correct various aberrations, to exert a large zoom ratio, and to be compact in size at low price.
Japanese Patent Application Publication No. 2007-79107(Reference 1), No. 2010-113150 (Reference 2), and Japanese Patent No. 4700953 (Reference 3) disclose such projector zoom lenses, for example.
The projector zoom lens in Reference 1 is high in zoom ratio of 1.3× and can correct various aberrations properly. However, it is comprised of 11 to 12 lenses and there is room for improvement in terms of compactness and lower price. With cost increases for lens materials or processing taken into account, it is important to reduce the total number of lens elements for the purpose of cost reduction of the projector zoom lens.
Further, the projector zoom lenses disclosed in References 2 and 3 are made up of 5 lenses and 7 lenses, respectively. However, the zoom ratios thereof are as low as 1.05 to 1.2×. Also, aberrations cannot be corrected sufficiently.